CytomX Therapeutics (CTMX) 10-Q 6 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Trading Symbol	CTMX
Entity Registrant Name	CytomX Therapeutics, Inc.
Entity Central Index Key	0001501989
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity File Number	001-37587
Entity Tax Identification Number	27-3521219
Entity Address, Address Line One	151 Oyster Point Blvd
Entity Address, Address Line Two	Suite 400
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	515-3185
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.00001 par value per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false
Entity Common Stock, Shares Outstanding		65,011,130

CONDENSED BALANCE SHEETS

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	[1]
Current assets:
Cash and cash equivalents	$ 329,666	$ 191,859
Short-term investments	64,130	124,260
Accounts receivable	756	798
Prepaid expenses and other current assets	5,456	7,096
Total current assets	400,008	324,013
Property and equipment, net	7,172	6,950
Intangible assets, net	1,130	1,167
Goodwill	949	949
Restricted cash	917	917
Operating lease right-of-use asset	21,736	22,495
Other assets	2,246	2,172
Total assets	434,158	358,663
Current liabilities:
Accounts payable	2,582	2,996
Accrued liabilities	18,831	23,059
Deferred revenue, current portion	76,636	74,869
Total current liabilities	98,049	100,924
Deferred revenue, net of current portion	169,280	186,261
Operating lease liabilities - long term	20,807	21,675
Total liabilities	288,136	308,860
Commitments and contingencies (Note 11)
Stockholders' equity:
Common stock, $0.00001 par value; 150,000,000 shares authorized and 65,002,897 and 48,251,819 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively	1	1
Additional paid-in capital	611,733	499,964
Accumulated other comprehensive loss	(43)	(47)
Accumulated deficit	(465,669)	(450,115)
Total stockholders' equity	146,022	49,803
Total liabilities and stockholders' equity	434,158	358,663
Convertible Preferred Stock
Stockholders' equity:
Convertible preferred stock, $0.00001 par value; 10,000,000 shares authorized and no shares issued and outstanding at March 31, 2021 and December 31, 2020.
[1]	The condensed balance sheet as of December 31, 2020 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	65,002,897	48,251,819
Common stock, shares outstanding	65,002,897	48,251,819
Convertible Preferred Stock
Convertible Preferred stock, par value	$ 0.00001	$ 0.00001
Convertible Preferred stock, shares authorized	10,000,000	10,000,000
Convertible Preferred stock, shares issued	0	0
Convertible Preferred stock, shares outstanding	0	0

CONDENSED STATEMENTS OF OPERATI

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 15,971	$ 49,593
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 22,371	$ 42,814
General and administrative	9,227	9,572
Total operating expenses	31,598	52,386
Loss from operations	(15,627)	(2,793)
Interest income	68	1,075
Other income, net	5	12
Loss before income taxes	(15,554)	(1,706)
Provision for (benefit from) income taxes		(13,911)
Net income (loss)	$ (15,554)	$ 12,205
Net income (loss) per share
Basic	$ (0.26)	$ 0.27
Diluted	$ (0.26)	$ 0.26
Shares used to compute net income (loss) per share
Basic	60,968,111	45,723,955
Diluted	60,968,111	47,044,774
Other comprehensive income:
Unrealized gain on short-term investments, net of tax	$ 4	$ 279
Comprehensive income (loss)	$ (15,550)	$ 12,484

STATEMENTS OF STOCKHOLDERS' EQU

STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total		Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2019	$ 51,113		$ 1	$ 468,285	$ 57	$ (417,230)
Beginning balance, shares at Dec. 31, 2019			45,523,088
Exercise of stock options	2,157			2,157
Exercise of stock options, shares			395,528
Stock-based compensation	4,013			4,013
Other comprehensive income	279				279
Net income (loss)	12,205					12,205
Ending balance at Mar. 31, 2020	69,767		$ 1	474,455	336	(405,025)
Ending balance, shares at Mar. 31, 2020			45,918,616
Beginning balance at Dec. 31, 2020	49,803	[1]	$ 1	499,964	(47)	(450,115)
Beginning balance, shares at Dec. 31, 2020			48,251,819
Issuance of common stock in follow-on offering, net of issuance costs	107,712			107,712
Issuance of common stock in follow-on offering, net of issuance costs, shares			16,428,571
Exercise of stock options	$ 1,023			1,023
Exercise of stock options, shares	322,507		322,507
Stock-based compensation	$ 3,034			3,034
Other comprehensive income	4				4
Net income (loss)	(15,554)					(15,554)
Ending balance at Mar. 31, 2021	$ 146,022		$ 1	$ 611,733	$ (43)	$ (465,669)
Ending balance, shares at Mar. 31, 2021			65,002,897
[1]	The condensed balance sheet as of December 31, 2020 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net income (loss)	$ (15,554)	$ 12,205
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Amortization of intangible assets	36	36
Depreciation and amortization	638	601
Amortization of premium (accretion of discounts) on investments	141	(127)
Stock-based compensation expense	3,034	4,013
Non-cash lease expense	759	700
Changes in operating assets and liabilities
Accounts receivable	42	(129,997)
Prepaid expenses and other current assets	1,640	(14,957)
Other assets	(74)	636
Accounts payable	(292)	52
Accrued and other long-term liabilities	(5,096)	(3,315)
Deferred revenue	(15,214)	80,416
Net cash used in operating activities	(29,940)	(49,737)
Cash flows from investing activities:
Purchases of property and equipment	(982)	(1,049)
Purchases of short-term investments		(44,707)
Maturities of short-term investments	59,994	48,197
Net cash provided by investing activities	59,012	2,441
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	107,712
Proceeds from exercise of stock options	1,023	2,157
Net cash provided by financing activities	108,735	2,157
Net increase (decrease) in cash, cash equivalents and restricted cash	137,807	(45,139)
Cash, cash equivalents and restricted cash, beginning of period	192,776	189,342
Cash, cash equivalents and restricted cash, end of period	$ 330,583	$ 144,203

Description of the Business

Description of the Business	3 Months Ended
	Mar. 31, 2021
Disclosure Text Block [Abstract]
Description of the Business	1. Description of the Business CytomX Therapeutics, Inc. (the “Company”) is a clinical-stage, oncology-focused biopharmaceutical company with a vision of transforming lives with safer, more effective therapies. The Company aims to build a commercial-stage enterprise to maximize its impact on the treatment of cancer. The Company is advancing potential first-in-class and best-in-class antibody-based therapeutics created using its Probody® therapeutic technology platform that could meaningfully improve outcomes for cancer patients. Its proprietary and unique Probody technology platform is designed to enable “conditional activation” of antibody-based drugs within the tumor while minimizing drug activity in healthy tissues and in circulation. The Company is located in South San Francisco, California and was incorporated in the state of Delaware in September 2010.

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	2. Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation The accompanying interim condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) regarding interim financial reporting. Unaudited Interim Financial Information The accompanying interim condensed financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the results of operations for the periods presented. The condensed balance sheet data, as of December 31, 2020, was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. The condensed results of operations for the three months ended March 31, 2021 are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period. The accompanying condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 filed with the SEC. Use of Estimates The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates. Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents. Restricted Cash Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that sum to the total of the amounts shown in the condensed statements of cash flows. March 31, 2021 December 31, 2020 March 31, 2020 December 31, 2019 (in thousands) Cash and cash equivalents $ 329,666 $ 191,859 $ 143,286 $ 188,425 Restricted cash - non-current assets 917 917 917 917 Total $ 330,583 $ 192,776 $ 144,203 $ 189,342 Short-term Investments All investments have been classified as available-for-sale (“AFS”) and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities less than 12 months are considered short-term investments. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold. The Company assesses impairment of its AFS debt securities investments at each reporting period. Unrealized gains resulting from the excess of the fair value over the amortized cost basis of an investment are reported as a component of accumulated other comprehensive income (loss), net of tax. Unrealized losses or impairments resulting from the fair value of the AFS debt security being below the amortized cost basis are evaluated, using the discounted cash flow model, for identification of credit losses and non-credit related losses. Any credit losses are charged to earnings against the allowance for credit losses of the security, limited to the difference between the fair value and the amortized cost basis of the security. Any difference between the fair value of the security and the amortized cost basis, less the allowance for credit losses, are reported in other comprehensive income (loss). Expected cash inflows due to improvements in credit are recognized through a reversal of the allowance for credit losses subject to the total allowance previously recognized. In the event of impairment of any security, if management (i) has the intent to sell such security or (ii) will more-likely-than-not be required to sell such security before recovery of its amortized cost basis, such AFS debt security’s amortized cost basis will be written down to its fair value through earnings along with any existing allowance for credit losses. Comprehensive Income (Loss) Comprehensive income (loss) represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s non-credit related unrealized gains and losses on short-term investments during the period represents the component of other comprehensive income (loss) that is excluded from the reported net loss. Revenue Recognition The Company ’ ’ The Company assesses whether the promises in its arrangements with customers are considered as distinct performance obligations that should be accounted for separately. Judgment is required to determine whether the license to the Company ’ The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones. Such milestone payments are typically payable under the collaborations when the collaboration partner claims or selects a target, or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities or upon receipt of actual marketing approvals of a covered product or for additional indications. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At each reporting date, the Company re-evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price by using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price in such period of determination. The Company ’ The transaction price in each arrangement is allocated to the identified performance obligations based on the relative standalone selling price ( “ ” ’ In certain cases, the Company’s performance creates an asset that does not have an alternative use to the customer and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. AbbVie Ireland Unlimited Company ( “ ” ’ “ ” ’ Contract Balances Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company satisfies its performance obligations under these arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional. Research and Development Expenses The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and includes these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred. Stock-based The Company recognizes compensation costs related to stock options granted to employees based on the The Company estimates the fair value of its stock-based awards using the Black-Scholes option-pricing • Expected term. The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on vesting terms, exercise term and contractual lives of the options. The expected term of the ESPP shares is equal to the six-month look-back period. • Expected volatility. The expected stock price volatility for the Company’s stock options is based on the historical stock price volatility over the period which is commensurate with the estimated expected term of the stock awards. Volatility for ESPP shares is equal to the Company’s historical volatility over a six-month offering period. • Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury yield with a maturity equal to the expected term of the stock options in effect at the time of grant. • Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and has no current plan to pay any dividends on its common stock. Leases The Company determines if an arrangement is or contains a lease at inception. Operating leases are recorded as operating lease right-of-use (“ROU”) assets and operating lease liabilities in the Company’s balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses an implicit rate when readily available, or its incremental borrowing rate based on the information available at lease commencement date in determining the present value of lease payments. The operating lease ROU assets also include any lease prepayments made and reduced by lease incentives. The Company’s lease terms may include options to extend the lease when it is reasonably certain that such option will be exercised. Lease expenses are recognized on a straight-line basis over the lease term. The Company elected the short-term lease recognition exemption . Adopted Accounting Pronouncements In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Net Income (Loss) Per Share

Net Income (Loss) Per Share	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	3. Net Income (Loss) Per Share Basic net income (loss) per share is calculated by dividing the net income (loss) by the weighted-average number of shares of common stock outstanding for the period. Diluted net income per share is calculated using the weighted-average number of common shares outstanding, plus potential dilutive common stock during the period. Diluted net loss per share, for the comparative period, is the same as basic net loss per share since the effect of the potentially dilutive securities is anti-dilutive. The following table presents the calculation of basic and diluted net income (loss) per share: Three Months Ended March 31, 2021 2020 (in thousands) Numerator: Net income (loss) $ (15,554 ) $ 12,205 Denominator: Basic Weighted-average common shares used to calculate net income (loss) per share 60,968,111 45,723,955 Diluted Weighted-average common shares used to calculate net income (loss) per share 60,968,111 45,723,955 Effect of potentially dilutive securities: Stock options and ESPP shares - 1,320,819 60,968,111 47,044,774 Net income (loss) per share Basic $ (0.26 ) $ 0.27 Diluted $ (0.26 ) $ 0.26 The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net income (loss) per share for the periods presented, because including them would have been anti-dilutive: Three Months Ended March 31, 2021 2020 Options and ESPP to purchase common stock 11,554,985 8,234,298

Fair Value Measurements and Sho

Fair Value Measurements and Short-Term Investments	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements and Short-Term Investments	4. Fair Value Measurements and Short-Term Investments In accordance with Accounting Standards Codification (“ASC”) 820-10, Fair Value Measurements and Disclosures, the Company determines the fair value of financial and non-financial assets and liabilities using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows: • Level I: Inputs which include quoted prices in active markets for identical assets and liabilities. • Level II: Inputs other than Level I that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. • Level III: Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. The carrying amounts of the Company’s financial instruments, including restricted cash, accounts receivable, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. The Company’s financial instruments consist of Level I assets which consist primarily of highly liquid money market funds, some of which are included in restricted cash, and U.S. government bonds that are included in short-term investments. The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements: March 31, 2021 Valuation Hierarchy Amortized Cost Allowance for Credit Losses Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 281,201 $ — $ — $ — $ 281,201 Restricted cash (money market funds) Level I 917 — — — 917 U.S. Government bonds Level I 64,120 — 10 — 64,130 Total securities $ 346,238 $ — $ 10 $ — $ 346,248 December 31, 2020 Valuation Hierarchy Amortized Cost Allowance for Credit Losses Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 131,121 $ — $ — $ — $ 131,121 Restricted cash (money market funds) Level I 917 — — — 917 U.S. Government bonds Level I 124,254 — 6 — 124,260 Total securities $ 256,292 $ — $ 6 $ — $ 256,298 No securities have contractual maturities of greater than twelve months.

Accrued Liabilities

Accrued Liabilities	3 Months Ended
	Mar. 31, 2021
Accrued Liabilities Current [Abstract]
Accrued Liabilities	5. Accrued Liabilities Accrued liabilities consisted of the following: March 31, December 31, 2021 2020 (in thousands) Research and clinical expenses $ 8,486 $ 10,092 Payroll and related expenses 5,043 8,362 Legal and professional expenses 1,189 815 Operating lease liabilities - short term 3,297 3,195 Other accrued expenses 816 595 Total $ 18,831 $ 23,059

Research and Collaboration Agre

Research and Collaboration Agreements	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	6. Research and Collaboration Agreements The following table summarizes the revenue by collaboration partners: Three Months Ended March 31, 2021 2020 (in thousands) AbbVie $ 1,228 $ 30,096 Amgen 2,582 1,695 Astellas 4,756 387 Bristol Myers Squibb 7,405 17,415 Total revenue $ 15,971 $ 49,593 AbbVie Ireland Unlimited Company In April 2016, the Company and AbbVie entered into two agreements, a CD71 Co-Development and Licensing Agreement (the “ ” “ ” “ ” “A Under the CD71 Agreement, the Company received an upfront payment of $20.0 million in April 2016, and was eligible to initially receive up to $470.0 million in development, regulatory and commercial milestone payments, a 35% profit split on U.S. sales, and royalties on ex-U.S. sales at percentages in the high teens to low twenties if the Company participates in the co-development of the CD71 conditionally activated ADC subject to a reversion to a royalty on U.S. sales, and reduction in royalties on ex-U.S. sales, if the Company opts-out from the co-development of the CD71 conditionally activated ADC. The Company ’ ’ “ ” Under the terms of the Discovery Agreement, AbbVie receives exclusive worldwide rights to develop and commercialize conditionally activated ADCs against up to two targets, one of which was selected in March 2017. The Company shall perform research services to discover the Probody therapeutics and create conditionally activated ADCs for the nominated collaboration targets. From that point, AbbVie shall have sole right and responsibility for development and commercialization of products comprising or containing such conditionally activated ADCs ( “ ” Under the Discovery Agreement, the Company received an upfront payment of $10.0 million in April 2016 and subsequently earned an additional $10.0 million milestone payment triggered by selection of the second target by AbbVie in June 2019. The Company is also eligible to receive up to $265.0 million for each target, in development, regulatory and commercial milestone payments and royalties at percentages in the high single to low teens from commercial sales of any resulting conditionally activated ADCs. The second target was selected under the Discovery Agreement that allows AbbVie to select a target for developing a conditionally activated ADC or a Probody. The $40.0 million milestone payment earned in March 2020 for satisfying the CD71 dose escalation success criteria under the CD71 Agreement was included as part of the transaction price as it was unconstrained during the first quarter of 2020 and $26.6 million was recognized as revenue related to this milestone reflecting the percentage completed to-date on the project as of March 2020. The remainder is being recognized as revenues over the remaining estimated research service period through September 2022. The Company determined that the remaining potential milestone payments of both agreements, if recognized, are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company ’ The Company recognized revenue of $1.2 million and $30.1 million for the three months ended March 31, 2021 and 2020, respectively, related to the AbbVie Agreements. As of March 31, 2021 and December 31, 2020, deferred revenue related to the CD71 Agreement performance obligation was $24.7 million and $25.2 million, respectively, and deferred revenue related to the Discovery Agreement performance obligation was $7.3 million and $8.0 million, respectively. No amounts were due from AbbVie as of March 31, 2021 and December 31, 2020. Amgen, Inc. On September 29, 2017, the Company and Amgen, Inc. ( “ ” “ ” Concurrent with the entry into the Amgen Agreement, the Company and Amgen entered into a Share Purchase Agreement (the “ Purchase Agreement ” ) pursuant to which Amgen purchased 1,156,069 shares of the Company ’ s common stock at a price of $ 17.30 per share (calculated based on a 20-day volume-weighted average price), for total proceeds of $ 20.0 million, which the Company received on October 6, 2017, the closing date of the transaction. The Company estimated a premium on the stock sold to Amgen of $ 0.5 million, which takes into account a discount due to the lack of marketability resulting from the six-month lockup period. Under the terms of the Amgen Agreement, the Company and Amgen will co-develop a conditionally activated T-cell engaging bi-specific therapeutic targeting epidermal growth factor receptor (the “ ” “ ” Amgen also has the right to select a total of up to three targets, including the two additional targets discussed below. The Company and Amgen collaborate in the research and development of conditionally activated T-cell engaging bi-specifics products directed against such targets. Amgen has selected one such target (the “ ” “ ” “ ” ’ At the initiation of the collaboration, CytomX had the option to select, from programs specified in the Amgen Agreement, an existing preclinical stage T-cell engaging bispecific product from the Amgen preclinical pipeline. In March 2018, CytomX selected the program. CytomX is responsible, at its expense, for converting this program to a conditionally activated T-cell engaging bispecific product, and thereafter, will be responsible for development, manufacturing, and commercialization of the product ( “ ” The Company recognized revenue of $2.6 million and $1.7 million for the three months ended March 31, 2021 and 2020, respectively, related to the Amgen Agreement. As of March 31, 2021 and December 31, 2020, deferred revenue related to the EGFR Products performance obligation was $27.4 million and $29.8 million, respectively. As of March 31, 2021 and December 31, 2020, deferred revenue related to the Amgen Other Products performance obligation was $2.0 million and $2.2 million, respectively. No amounts were due from Amgen as of March 31, 2021 and December 31, 2020. Astellas Pharma Inc. The Company and Astellas Pharma, Inc. (“Astellas”) entered into a Collaboration and License Agreement (the “Astellas Agreement”) on March 23, 2020, the effective date, to collaborate on preclinical research activities to discover and develop certain antibody compounds for the treatment of cancer using the Company’s Probody therapeutic technology. Under the terms of the Astellas Agreement, the Company granted Astellas an exclusive, worldwide, rights to develop and commercialize Probody therapeutics for up to four collaboration targets including one initial target and three additional targets (“Additional Targets”). In addition, Astellas has the right to expand the number of Additional Targets from three up to five (the “Expansion Option”) before the third anniversary of the effective date. Furthermore, for a specified number of targets, at a pre-specified time prior to the initiation of the first pivotal study of a product against such target, the Company may elect to participate in certain development costs and share in the profits generated in the United States with respect to such product (“Cost Share Option”). The Cost Share Option, if exercised, will also provide the option for the Company to co-commercialize such product in the United States. The Company does not consider the Cost Share Option as a performance obligation at the inception of the agreement as the participation is at the Company’s discretion. Pursuant to the Astellas Agreement, the consideration from Astellas is comprised of an upfront fee of $80.0 million and contingent payments for development, regulatory and sales milestones of up to an aggregate of approximately $1.6 billion. If Astellas exercises its Expansion Option for the two Additional Targets, the Company would be eligible to receive additional upfront and milestone payments aggregating to approximately $0.9 billion. The Company recognized revenue of $4.8 million and $0.4 million for the three months ended March 31, 2021 and 2020, respectively. As of March 31, 2021 and December 31, 2020, deferred revenue relating to the Astellas Agreement was $63.6 million and $67.6 million, respectively. The amount due from Astellas under the Astellas Agreement was $0.8 million and $0.8 million, as of March 31, 2021 and December 31, 2020, respectively. Bristol Myers Squibb Company On May 23, 2014, the Company and Bristol Myers Squibb Company ( “ ” “ ” ’ Under the terms of the BMS Agreement, the Company granted Bristol Myers Squibb exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. Bristol Myers Squibb had additional rights to substitute up to two collaboration targets within three years of the effective date of the BMS Agreement. These rights expired in May 2017. Each collaboration target had a two-year Pursuant to the BMS Agreement, the financial consideration from Bristol Myers Squibb was comprised of an upfront payment of $50.0 million and the Company was initially entitled to receive contingent payments of up to $25.0 million for additional targets and up to an aggregate of $1,192.0 million for development, regulatory and sales milestones. In addition, the Company was entitled to royalty payments in the mid-single digits to low double-digit percentages from potential future sales. The Company also received research and development service fees based on a prescribed FTE rate that was capped. On March 17, 2017, the Company and Bristol Myers Squibb entered into Amendment Number 1 to Extend Collaboration and License Agreement ( “ ” “ ” Pursuant to Amendment 1, the financial consideration from Bristol Myers Squibb was comprised of an upfront payment of $200.0 million and the Company was initially eligible to receive contingent payments for development, regulatory and sales milestones of up to an aggregate of $3,586.0 million for the eight targets. The Company was also entitled to tiered mid-single to low double-digit percentage royalties from potential future sales. Amendment 1 did not change the term of the Bristol Myers Squibb ’ ’ The initial transaction price for the BMS Agreement and Amendment 1, collectively, was $272.8 million consisting of the upfront fees of $250.0 million, research and development service fees of $10.8 million and milestone payments received to date of $12.0 million. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company ’ In February 2020, Bristol Myers Squibb dosed the first patient in the Part 2 cohort expansion portion of its ongoing BMS-986249 clinical study for the CTLA-4 program, which triggered a $10.0 million milestone payment to the Company In February 2021, the Company and Bristol Myers Squibb entered into Amendment Number 2 to amend the Collaboration and License Agreement (“Amendment 2”), as amended by Amendment 1. Subsequent to Amendment 2, Bristol Myers Squibb has the exclusive worldwide rights to develop and commercialize Probody therapeutics for up to five oncology targets. Under the terms of Amendment 2, the period for target selection has been extended and the Company will continue to collaborate with Bristol Myers Squibb to discover and conduct preclinical development of Probody therapeutics against targets selected by Bristol Myers Squibb over the estimated research period, which ends in April 2025. Pursuant to Amendment 2, the Company is eligible to receive contingent payments for development, regulatory and sales milestones of up to an aggregate of $1,779.0 million. It is also entitled to tiered mid-single to low double-digit percentage of royalties from potential future sales. In addition, the Company will no longer be entitled to receive the research and development service fee as part of the arrangement. The Company re-evaluated the remaining potential milestone payments and determined that, if recognized, significant revenue reversal was still probable as the achievement of such milestones was highly dependent on factors outside the Company ’ The Company recognized revenue of $7.4 million and $17.4 million for the three months ended March 31, 2021 and 2020, respectively. As of March 31, 2021 and December 31, 2020, deferred revenue relating to the BMS Agreement was $120.9 million and $128.3 million, respectively. No amounts were due from Bristol Myers Squibb as of March 31, 2021 and December 31, 2020. ImmunoGen, Inc. In January 2014, the Company and ImmunoGen, Inc. ( “ ” “ ” ’ “ ” ’ ’ ’ praluzatamab ravtansine ( “ ” In December 2019, the parties entered into a license agreement (the “ImmunoGen 2019 License”) pursuant to which the ImmunoGen Research Agreement (the ”ImmunoGen 2017 License”) for a target in December 2017, was terminated and ImmunoGen granted a license for all of ImmunoGen’s rights under the ImmunoGen 2017 License to the Company. In February 2020, the Company dosed the first patient in the praluzatamab ravtansine Phase 2 clinical trial and triggered a $3.0 million milestone payment to ImmunoGen pursuant to the CX-2009 License which continued to remain in effect following the termination of the ImmunoGen 2017 License in December 2019. The Company recorded a $3.0 million charge to research and development expense for the three months ended March 31, 2020, in connection with this milestone payment to ImmunoGen. Contract Liabilities The following table presents changes in the Company’s total contract liabilities for the three months ended March 31, 2021: Balance at 12/31/2020 Additions Deductions Balance at 3/31/2021 (in thousands) Contract liabilities: Deferred revenue $ 261,130 $ — $ (15,215 ) $ 245,916 The Company expects that the $245.9 million of deferred revenue related to the following contracts as of March 31, 2021 will be recognized as revenue as set forth below. However, the timing of revenue recognition could differ from the estimates depending on facts and circumstances impacting the various contracts, including progress of research and development, resources assigned to the contracts by the Company or its collaboration partners, or other factors outside of the Company’s control. • The $24.7 million of deferred revenue related to the CD71 Agreement as of March 31, 2021 is expected to be recognized based on actual FTE effort and program progress until approximately September 2022. • The $0.8 million of deferred revenue related to the first target under the Discovery Agreement as of March 31, 2021 is expected to be recognized ratably until approximately April 2022. • The $6.5 million of deferred revenue related to the second target under the Discovery Agreement as of March 31, 2021 is expected to be recognized ratably until approximately June 2024. • The $27.4 million of deferred revenue related to the Amgen EGFR Products as of March 31, 2021 is expected to be recognized based on actual FTE effort and program progress until approximately September 2024. • The $2.0 million of deferred revenue related to the Amgen Other Products as of March 31, 2021 is expected to be recognized ratably until approximately September 2023. • The $63.6 million of deferred revenue related to the Astellas Agreement as of March 31, 2021 is expected to be recognized ratably until approximately March 2025. • The $120.9 million of deferred revenue related to the BMS Agreement as of March 31, 2021 is expected to be recognized ratably until approximately April 2025.

License Agreements

License Agreements	3 Months Ended
	Mar. 31, 2021
Research And Development [Abstract]
License Agreements	7. License Agreements UCSB The Company has an exclusive, worldwide license agreement with UCSB (the “ ” In February 2020, the Company recorded $0.8 million of sublicense fees triggered by the $10.0 million milestone payment from Bristol Myers Squibb’s dosing of the first patient in the Part 2 cohort expansion portion of its ongoing BMS-986249 clinical study for the CTLA-4 program. In March 2020, the Company incurred additional sublicense fees of $6.0 million related to the $80.0 million upfront fee received pursuant to the Astellas Agreement entered into in March 2020, and $1.4 million related to the $40.0 million milestone payment from AbbVie for satisfying the CD71 dose escalation success criteria under the CD71 Agreement in March 2020. During the three months ended March 31, 2021 and 2020, the Company incurred sublicense expenses of $0.9 million and $9.1 million, respectively under the provisions of the UCSB Agreement. As of March 31, 2021 and December 31, 2020, there was no outstanding sublicense fee payable to UCSB.

Common Stock

Common Stock	3 Months Ended
	Mar. 31, 2021
Equity [Abstract]
Common Stock	8. Common Stock In January 2021, the Company completed an underwritten public offering of 14,285,714 shares of common stock at a price of $7.00 per share. The aggregate net proceeds received by the Company from the offering were approximately $93.6 million, after deducting underwriting discounts and commissions and offering expenses of $6.4 million. The Company also granted the underwriters the option, for 30 days, to purchase up to 2,142,857 additional shares of common stock at the public offering price, less the underwriting discounts and commissions. In February 2021, the underwriters exercised the option in full which resulted in additional net proceeds of $14.1 million to the Company, after deducting the underwriting discounts and commissions of $0.9 million.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation Stock Options Activities under the Company’s stock option plans for the three months ended March 31, 2021 were as follows: Options Outstanding Number of Options Weighted- Average Exercise Price Per Share Balances at December 31, 2020 10,929,530 $ 10.77 Options granted 2,792,347 7.87 Options exercised (322,507 ) 3.17 Option forfeited/expired (1,535,118 ) 11.15 Balances at March 31, 2021 11,864,252 $ 10.25 Options exercisable at March 31, 2021 5,341,720 $ 12.11 Stock-based Compensation Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows: Three Months Ended March 31, 2021 2020 (in thousands) Stock-based compensation expense: Research and development $ 1,337 $ 1,919 General and administrative 1,697 2,094 Total stock-based compensation expense $ 3,034 $ 4,013

Leases

Leases	3 Months Ended
	Mar. 31, 2021
Leases [Abstract]
Leases	10. Leases Operating Lease On December 10, 2015, the Company entered into a lease (the “2016 Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,000 rentable square feet of office and laboratory space located in South San Francisco, California for the Company’s new corporate headquarters. The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease. The Lease provided for annual base rent of approximately $3.1 million in the first year of the lease term. The annual base rent for the second twelve months was approximately $4.3 million, which will increase on an annual basis beginning from the 25 th In addition, the Company obtained a standby letter of credit (the “Letter of Credit”) in an amount of approximately $0.9 million, which may be drawn by the Landlord to be applied for certain purposes upon the Company’s breach of any provisions under the 2016 Lease. The Company has recorded the $0.9 million Letter of Credit as non-current restricted cash on its balance sheet as of March 31, 2021 and December 31, 2020, respectively. Rent expense is recognized on a straight-line basis over the term of the lease and accordingly the Company records the difference between cash rent payments and the recognition of rent expense against the operating lease ROU asset. Rent expense during each of the three months ended March 31, 2021 and 2020 was $1.3 million. Supplemental information related to leases are as follows: Three Months Ended March 31, 2021 March 31, 2020 (in thousands) Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 1,273 $ 1,239 March 31, 2021 December 31, 2020 (in thousands) Supplemental balance sheet information related to leases: Operating lease right-of-use assets $ 21,736 $ 22,495 Current operating lease liabilities 3,297 3,195 Non-current operating lease liabilities 20,807 21,675 Total operating lease liabilities $ 24,104 $ 24,870 Weighted-average remaining lease term (in years) Operating lease 5.50 5.75 Weighted-average discount rate Operating lease 8.25 % 8.25 % March 31, 2021 (in thousands) Maturity of operating lease liabilities 2021 $ 3,856 2022 5,273 2023 5,420 2024 5,572 2025 and beyond 10,116 Total lease payments 30,237 Less imputed interest (6,133 ) Present value of lease liabilities $ 24,104

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Legal Proceedings On March 4, 2020, Vytacera Bio, LLC filed a patent infringement lawsuit against the Company in the U.S. District Court for the District of Delaware. The lawsuit alleges that the Company’s use, offers to sell, and/or sales of the Probody® technology platform for basic research applications constitutes infringement. The complaint seeks unspecified monetary damages. The Company filed an Answer, Affirmative Defenses, and Counterclaims on May 26, 2020. Vytacera Bio, LLC filed its Answer to CytomX Therapeutics Inc.’s Counterclaims on June 5, 2020. The parties have agreed to a case schedule. Discovery is ongoing. The Company believes that the lawsuit is without merit and intends to vigorously defend itself and has not recorded any amount for claims associated with this lawsuit as of March 31, 2021 .

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes The Company records the effect of an enacted change in a tax law in the period that includes the enactment date in accordance with ASC 740. On March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) was enacted in response to the COVID-19 pandemic. The tax relief measures under the CARES Act for businesses include a five-year The income tax benefit during the three months ended March 31, 2020 was generated as a result of the recognition of net operating loss carryback under the CARES Act which generated a refund of income taxes paid for 2018. The Company maintains a full valuation allowance against its net deferred tax assets due to the Company’s history of losses as of March 31, 2021 and December 31, 2020 .

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying interim condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) regarding interim financial reporting.
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying interim condensed financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the results of operations for the periods presented. The condensed balance sheet data, as of December 31, 2020, was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. The condensed results of operations for the three months ended March 31, 2021 are not necessarily indicative of the results to be expected for the full year or for any other future year or interim period. The accompanying condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 filed with the SEC.
Use of Estimates	Use of Estimates The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.
Restricted Cash	Restricted Cash Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that sum to the total of the amounts shown in the condensed statements of cash flows. March 31, 2021 December 31, 2020 March 31, 2020 December 31, 2019 (in thousands) Cash and cash equivalents $ 329,666 $ 191,859 $ 143,286 $ 188,425 Restricted cash - non-current assets 917 917 917 917 Total $ 330,583 $ 192,776 $ 144,203 $ 189,342
Short-term Investments	Short-term Investments All investments have been classified as available-for-sale (“AFS”) and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities less than 12 months are considered short-term investments. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold. The Company assesses impairment of its AFS debt securities investments at each reporting period. Unrealized gains resulting from the excess of the fair value over the amortized cost basis of an investment are reported as a component of accumulated other comprehensive income (loss), net of tax. Unrealized losses or impairments resulting from the fair value of the AFS debt security being below the amortized cost basis are evaluated, using the discounted cash flow model, for identification of credit losses and non-credit related losses. Any credit losses are charged to earnings against the allowance for credit losses of the security, limited to the difference between the fair value and the amortized cost basis of the security. Any difference between the fair value of the security and the amortized cost basis, less the allowance for credit losses, are reported in other comprehensive income (loss). Expected cash inflows due to improvements in credit are recognized through a reversal of the allowance for credit losses subject to the total allowance previously recognized. In the event of impairment of any security, if management (i) has the intent to sell such security or (ii) will more-likely-than-not be required to sell such security before recovery of its amortized cost basis, such AFS debt security’s amortized cost basis will be written down to its fair value through earnings along with any existing allowance for credit losses.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s non-credit related unrealized gains and losses on short-term investments during the period represents the component of other comprehensive income (loss) that is excluded from the reported net loss.
Revenue Recognition	Revenue Recognition The Company ’ ’ The Company assesses whether the promises in its arrangements with customers are considered as distinct performance obligations that should be accounted for separately. Judgment is required to determine whether the license to the Company ’ The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones. Such milestone payments are typically payable under the collaborations when the collaboration partner claims or selects a target, or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities or upon receipt of actual marketing approvals of a covered product or for additional indications. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At each reporting date, the Company re-evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price by using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price in such period of determination. The Company ’ The transaction price in each arrangement is allocated to the identified performance obligations based on the relative standalone selling price ( “ ” ’ In certain cases, the Company’s performance creates an asset that does not have an alternative use to the customer and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. AbbVie Ireland Unlimited Company ( “ ” ’ “ ” ’
Contract Balances	Contract Balances Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company satisfies its performance obligations under these arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional.
Research and Development Expenses	Research and Development Expenses The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and includes these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred.
Stock-based Compensation	Stock-based The Company recognizes compensation costs related to stock options granted to employees based on the The Company estimates the fair value of its stock-based awards using the Black-Scholes option-pricing • Expected term. The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on vesting terms, exercise term and contractual lives of the options. The expected term of the ESPP shares is equal to the six-month look-back period. • Expected volatility. The expected stock price volatility for the Company’s stock options is based on the historical stock price volatility over the period which is commensurate with the estimated expected term of the stock awards. Volatility for ESPP shares is equal to the Company’s historical volatility over a six-month offering period. • Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury yield with a maturity equal to the expected term of the stock options in effect at the time of grant. • Dividend yield. The expected dividend is assumed to be zero as the Company has never paid dividends and has no current plan to pay any dividends on its common stock.
Leases	Leases The Company determines if an arrangement is or contains a lease at inception. Operating leases are recorded as operating lease right-of-use (“ROU”) assets and operating lease liabilities in the Company’s balance sheet. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses an implicit rate when readily available, or its incremental borrowing rate based on the information available at lease commencement date in determining the present value of lease payments. The operating lease ROU assets also include any lease prepayments made and reduced by lease incentives. The Company’s lease terms may include options to extend the lease when it is reasonably certain that such option will be exercised. Lease expenses are recognized on a straight-line basis over the lease term. The Company elected the short-term lease recognition exemption .
Adopted Accounting Pronouncements	Adopted Accounting Pronouncements In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Schedule of Cash, Cash Equivalents, and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed balance sheets that sum to the total of the amounts shown in the condensed statements of cash flows. March 31, 2021 December 31, 2020 March 31, 2020 December 31, 2019 (in thousands) Cash and cash equivalents $ 329,666 $ 191,859 $ 143,286 $ 188,425 Restricted cash - non-current assets 917 917 917 917 Total $ 330,583 $ 192,776 $ 144,203 $ 189,342

Net Income (Loss) Per Share (Ta

Net Income (Loss) Per Share (Tables)	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Summary of Calculation of Basic and Diluted Net Income (Loss) Per Share	The following table presents the calculation of basic and diluted net income (loss) per share: Three Months Ended March 31, 2021 2020 (in thousands) Numerator: Net income (loss) $ (15,554 ) $ 12,205 Denominator: Basic Weighted-average common shares used to calculate net income (loss) per share 60,968,111 45,723,955 Diluted Weighted-average common shares used to calculate net income (loss) per share 60,968,111 45,723,955 Effect of potentially dilutive securities: Stock options and ESPP shares - 1,320,819 60,968,111 47,044,774 Net income (loss) per share Basic $ (0.26 ) $ 0.27 Diluted $ (0.26 ) $ 0.26
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Income (Loss) Per Share	The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net income (loss) per share for the periods presented, because including them would have been anti-dilutive: Three Months Ended March 31, 2021 2020 Options and ESPP to purchase common stock 11,554,985 8,234,298

Fair Value Measurements and S_2

Fair Value Measurements and Short-Term Investments (Tables)	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis	The following tables set forth the fair value of the Company’s short-term investments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements: March 31, 2021 Valuation Hierarchy Amortized Cost Allowance for Credit Losses Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 281,201 $ — $ — $ — $ 281,201 Restricted cash (money market funds) Level I 917 — — — 917 U.S. Government bonds Level I 64,120 — 10 — 64,130 Total securities $ 346,238 $ — $ 10 $ — $ 346,248 December 31, 2020 Valuation Hierarchy Amortized Cost Allowance for Credit Losses Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value (in thousands) Assets Money market funds Level I $ 131,121 $ — $ — $ — $ 131,121 Restricted cash (money market funds) Level I 917 — — — 917 U.S. Government bonds Level I 124,254 — 6 — 124,260 Total securities $ 256,292 $ — $ 6 $ — $ 256,298

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	3 Months Ended
	Mar. 31, 2021
Accrued Liabilities Current [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following: March 31, December 31, 2021 2020 (in thousands) Research and clinical expenses $ 8,486 $ 10,092 Payroll and related expenses 5,043 8,362 Legal and professional expenses 1,189 815 Operating lease liabilities - short term 3,297 3,195 Other accrued expenses 816 595 Total $ 18,831 $ 23,059

Research and Collaboration Ag_2

Research and Collaboration Agreements (Tables)	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of Revenue by Collaboration Partners	The following table summarizes the revenue by collaboration partners: Three Months Ended March 31, 2021 2020 (in thousands) AbbVie $ 1,228 $ 30,096 Amgen 2,582 1,695 Astellas 4,756 387 Bristol Myers Squibb 7,405 17,415 Total revenue $ 15,971 $ 49,593
Summary of Contract Liabilities	The following table presents changes in the Company’s total contract liabilities for the three months ended March 31, 2021: Balance at 12/31/2020 Additions Deductions Balance at 3/31/2021 (in thousands) Contract liabilities: Deferred revenue $ 261,130 $ — $ (15,215 ) $ 245,916

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Activities Under Company's Stock Option Plans	Activities under the Company’s stock option plans for the three months ended March 31, 2021 were as follows: Options Outstanding Number of Options Weighted- Average Exercise Price Per Share Balances at December 31, 2020 10,929,530 $ 10.77 Options granted 2,792,347 7.87 Options exercised (322,507 ) 3.17 Option forfeited/expired (1,535,118 ) 11.15 Balances at March 31, 2021 11,864,252 $ 10.25 Options exercisable at March 31, 2021 5,341,720 $ 12.11
Total Stock-based Compensation Recognized	Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows: Three Months Ended March 31, 2021 2020 (in thousands) Stock-based compensation expense: Research and development $ 1,337 $ 1,919 General and administrative 1,697 2,094 Total stock-based compensation expense $ 3,034 $ 4,013

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2021
Leases [Abstract]
Summary of Supplemental Information Related to Leases	Supplemental information related to leases are as follows: Three Months Ended March 31, 2021 March 31, 2020 (in thousands) Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 1,273 $ 1,239 March 31, 2021 December 31, 2020 (in thousands) Supplemental balance sheet information related to leases: Operating lease right-of-use assets $ 21,736 $ 22,495 Current operating lease liabilities 3,297 3,195 Non-current operating lease liabilities 20,807 21,675 Total operating lease liabilities $ 24,104 $ 24,870 Weighted-average remaining lease term (in years) Operating lease 5.50 5.75 Weighted-average discount rate Operating lease 8.25 % 8.25 %
Schedule of Maturity of Operating Lease Liabilities	March 31, 2021 (in thousands) Maturity of operating lease liabilities 2021 $ 3,856 2022 5,273 2023 5,420 2024 5,572 2025 and beyond 10,116 Total lease payments 30,237 Less imputed interest (6,133 ) Present value of lease liabilities $ 24,104

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020		Mar. 31, 2020	Dec. 31, 2019
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 329,666	$ 191,859	[1]	$ 143,286	$ 188,425
Restricted cash - non-current assets	917	917	[1]	917	917
Total	$ 330,583	$ 192,776		$ 144,203	$ 189,342
[1]	The condensed balance sheet as of December 31, 2020 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details) - ASU 2019-12	Mar. 31, 2021
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2021
Change in accounting principle, accounting standards update, adopted [true false]	false
Change in accounting principle, accounting standards update, immaterial effect [true false]	true

Net Income (Loss) Per Share - S

Net Income (Loss) Per Share - Summary of Calculation of Basic and Diluted Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net income (loss)	$ (15,554)	$ 12,205
Basic
Weighted-average common shares used to calculate net income (loss) per share	60,968,111	45,723,955
Diluted
Weighted-average common shares used to calculate net income (loss) per share	60,968,111	45,723,955
Effect of potentially dilutive securities:
Stock options and ESPP shares		1,320,819
Weighted-average common shares, diluted	60,968,111	47,044,774
Net income (loss) per share
Basic	$ (0.26)	$ 0.27
Diluted	$ (0.26)	$ 0.26

Net Income (Loss) Per Share -_2

Net Income (Loss) Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Income (Loss) Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Options and ESPP to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	11,554,985	8,234,298

Fair Value Measurements and S_3

Fair Value Measurements and Short-Term Investments - Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020		Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 346,238	$ 256,292
Gross Unrealized Holding Gains	10	6
Aggregate Fair Value	346,248	256,298
Cash and cash equivalents	329,666	191,859	[1]	$ 143,286	$ 188,425
Level I | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	281,201	131,121
Level I | Restricted cash (money market funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted cash	917	917
Level I | U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term Investments, Amortized Cost	64,120	124,254
Short-term Investments, Gross Unrealized Holding Gains	10	6
Short-term Investments, Aggregate Fair Value	$ 64,130	$ 124,260
[1]	The condensed balance sheet as of December 31, 2020 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

Fair Value Measurements and S_4

Fair Value Measurements and Short-Term Investments - Additional Information (Details)	Mar. 31, 2021USD ($)
Fair Value Disclosures [Abstract]
Contractual maturities of greater than twelve months	$ 0

Accrued Liabilities - Summary o

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 8,486	$ 10,092
Payroll and related expenses	5,043	8,362
Legal and professional expenses	1,189	815
Operating lease liabilities - short term	3,297	3,195
Other accrued expenses	816	595
Total	$ 18,831	$ 23,059	[1]
[1]	The condensed balance sheet as of December 31, 2020 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

Research and Collaboration Ag_3

Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partners (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 15,971	$ 49,593
AbbVie
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	1,228	30,096
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	2,582	1,695
Astellas
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	4,756	387
Bristol Myers Squibb
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 7,405	$ 17,415

Research and Collaboration Ag_4

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)	1 Months Ended				3 Months Ended
	Mar. 31, 2020USD ($)	May 31, 2018USD ($)	Jul. 31, 2017USD ($)	Apr. 30, 2016USD ($)AgreementTarget	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)	Mar. 31, 2017Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement					$ 15,971,000	$ 49,593,000
Type of Revenue [Extensible List]					us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue					$ 245,916,000		$ 261,130,000
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement					$ 1,200,000	$ 30,100,000
Type of Revenue [Extensible List]					us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
AbbVie Ireland Unlimited Company | Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of collaboration agreements | Agreement				2
Percentage of net profits or net losses related to development and commercialization costs				35.00%
Amount due from agreement					$ 0		0
AbbVie Ireland Unlimited Company | CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received				$ 20,000,000
Milestone payment received	$ 40,000,000	$ 21,000,000	$ 14,000,000
Deferred revenue					$ 24,700,000		25,200,000
AbbVie Ireland Unlimited Company | CD71 Agreement | Transferred over Time
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement	$ 26,600,000
Type of Revenue [Extensible List]					us-gaap:LicenseAndServiceMember
AbbVie Ireland Unlimited Company | CD71 Agreement | U.S
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Profit split on U.S. sales				35.00%
AbbVie Ireland Unlimited Company | CD71 Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development, regulatory and commercial milestones				$ 470,000,000
AbbVie Ireland Unlimited Company | Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid		$ 4,000,000	$ 1,000,000
AbbVie Ireland Unlimited Company | Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received				10,000,000
Number of targets selected | Target								1
Deferred revenue					$ 7,300,000		$ 8,000,000
AbbVie Ireland Unlimited Company | Discovery Agreement | Second Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments received				$ 10,000,000
AbbVie Ireland Unlimited Company | Discovery Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets | Target				2
AbbVie Ireland Unlimited Company | Discovery Agreement | Maximum | Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable				$ 265,000,000

Research and Collaboration Ag_5

Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)	Sep. 29, 2017USD ($)Target	Oct. 31, 2017USD ($)$ / sharesshares	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, value of shares issued in connection with agreement			$ 107,712,000
Revenue recognized from collaborative arrangement			$ 15,971,000	$ 49,593,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			$ 245,916,000		$ 261,130,000
Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			27,400,000
Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			2,000,000
Collaboration and License Agreement | Amgen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		$ 40,000,000
Common stock, shares issuable under agreement | shares		1,156,069
Common stock, shares issuable under agreement, price per share | $ / shares		$ 17.30
Common stock, value of shares issued in connection with agreement		$ 20,000,000
Period used to calculate weighted average price per share		20 days
Lock-up period for share disposal		6 months
Estimated premium on the stock sold		$ 500,000
Number of targets selected | Target	1
Number of additional collaboration target | Target	2
Revenue recognized from collaborative arrangement			$ 2,600,000	$ 1,700,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Amount due from agreement			$ 0		$ 0
Collaboration and License Agreement | Amgen Inc | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments payable	$ 203,000,000
Collaboration and License Agreement | Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			27,400,000		29,800,000
Collaboration and License Agreement | Amgen Inc | EGFR Products | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 455,000,000
Percentage share of profit and losses	50.00%
Collaboration and License Agreement | Amgen Inc | Amgen Products | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 950,000,000
Number of targets | Target	3
Collaboration and License Agreement | Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			$ 2,000,000		$ 2,200,000

Research and Collaboration Ag_6

Research and Collaboration Agreements - Astellas Pharma Inc - Additional Information (Details)	Mar. 23, 2020USD ($)Target	Mar. 31, 2020USD ($)	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement			$ 15,971,000	$ 49,593,000
Deferred revenue			245,916,000		$ 261,130,000
Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			63,600,000
Collaboration and License Agreement | Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets selected | Target	1
Number of additional collaboration target | Target	3
Upfront payment received	$ 80,000,000	$ 80,000,000
Revenue recognized from collaborative arrangement			4,800,000	$ 400,000
Deferred revenue			63,600,000		67,600,000
Amount due from customer			$ 800,000		$ 800,000
Collaboration and License Agreement | Astellas Pharma Inc. | Additional Contingent Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 900,000,000
Collaboration and License Agreement | Astellas Pharma Inc. | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets | Target	4
Right to expand the number of additional collaboration target | Target	5
Contingent milestone payments receivable	$ 1,600,000,000
Collaboration and License Agreement | Astellas Pharma Inc. | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Right to expand the number of additional collaboration target | Target	3

Research and Collaboration Ag_7

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)	Apr. 25, 2017USD ($)Target	Jul. 07, 2014USD ($)TargetTerm	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Feb. 28, 2021USD ($)	Dec. 31, 2020USD ($)	Feb. 29, 2020USD ($)	Mar. 17, 2017Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement			$ 15,971,000	$ 49,593,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			$ 245,916,000			$ 261,130,000
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			$ 120,900,000
Collaborative Arrangement | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration target research term		2 years
Number of additional collaboration target | Target		2
Research terms			Each collaboration target had a two-year research term and the two additional targets had to be nominated by Bristol Myers Squibb within five years of the effective date of the BMS Agreement. The research term for each collaboration target could be extended in one year increments up to three times.
Extension of research term for each collaboration target		1 year
Upfront payment received	$ 200,000,000	$ 50,000,000
Contingent milestone payments receivable					$ 1,779,000,000		$ 10,000,000
Number of research targets selected | Target	8
Total transaction price		272,800,000
Upfront payment received		250,000,000
Research and development service fees		10,800,000
Milestone payment received		$ 12,000,000		$ 10,000,000
Estimated research service termination date		Apr. 25, 2025
Revenue recognized from collaborative arrangement			$ 7,400,000	$ 17,400,000
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			$ 120,900,000			128,300,000
Amount due from agreement			$ 0			$ 0
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target | Target		4
Number of collaboration target | Target		2
Period of nomination of additional target from effective date		5 years
Times of increments for extended collaboration target research time | Term		3
Contingent milestone payments receivable	$ 3,586,000,000	$ 1,192,000,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Each Of Two Additional Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 25,000,000
Collaboration and License Agreement | Maximum | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of additional collaboration target | Target								8

Research and Collaboration Ag_8

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details)	1 Months Ended	3 Months Ended
	Jan. 31, 2014USD ($)Target	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Feb. 29, 2020USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Charge to research and development expense		$ 22,371,000	$ 42,814,000
Collaborative Arrangement | Immuno Gen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	$ 0
Number of research targets selected | Target	2
Milestone payments payable				$ 3,000,000
Charge to research and development expense			$ 3,000,000

Research and Collaboration Ag_9

Research and Collaboration Agreements - Summary of Contract Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Revenue From Contract With Customer [Abstract]
Balance at 12/31/2020	$ 261,130
Deductions	(15,215)
Balance at 3/31/2021	$ 245,916

Research and Collaboration A_10

Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 245,916	$ 261,130
AbbVie Ireland Unlimited Company | CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 24,700	$ 25,200
Deferred revenue recognition maturity month and year	2022-09
AbbVie Ireland Unlimited Company | First Target under Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 800
Deferred revenue recognition maturity month and year	2022-04
AbbVie Ireland Unlimited Company | Second Target under Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 6,500
Deferred revenue recognition maturity month and year	2024-06
Amgen Inc | EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 27,400
Deferred revenue recognition maturity month and year	2024-09
Amgen Inc | Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 2,000
Deferred revenue recognition maturity month and year	2023-09
Astellas Pharma Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 63,600
Deferred revenue recognition maturity month and year	2025-03
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 120,900
Deferred revenue recognition maturity month and year	2025-04

License Agreements - Additional

License Agreements - Additional Information (Details) - USD ($)	Mar. 23, 2020	Mar. 31, 2020	Feb. 29, 2020	May 31, 2018	Jul. 31, 2017	Apr. 30, 2016	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
License Agreement [Line Items]
Accrued liabilities							$ 18,831,000		$ 23,059,000	[1]
UCSB
License Agreement [Line Items]
Sublicense fees							900,000	$ 9,100,000
Accrued liabilities							$ 0		$ 0
Bristol Myers Squibb Company | Collaboration and License Agreement
License Agreement [Line Items]
Sublicense fees			$ 800,000
Milestone payment receivable			$ 10,000,000
Astellas Pharma Inc. | Collaboration and License Agreement
License Agreement [Line Items]
Additional sublicense fees		$ 6,000,000
Upfront payment received	$ 80,000,000	80,000,000
AbbVie Ireland Unlimited Company | CD71 Agreement
License Agreement [Line Items]
Additional sublicense fees		1,400,000
Upfront payment received						$ 20,000,000
Milestone payment received		$ 40,000,000		$ 21,000,000	$ 14,000,000
[1]	The condensed balance sheet as of December 31, 2020 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

Common Stock - Additional Infor

Common Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended
	Feb. 28, 2021	Jan. 31, 2021	Mar. 31, 2021
Class Of Stock [Line Items]
Common stock, value of shares issued in connection with agreement			$ 107,712
Underwritten Public Offering
Class Of Stock [Line Items]
Common stock, value of shares issued in connection with agreement		$ 93,600
Underwriters
Class Of Stock [Line Items]
Common stock, value of shares issued in connection with agreement	$ 14,100
Common Stock
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares			16,428,571
Common Stock | Underwritten Public Offering
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares		14,285,714
Common stock price, per share		$ 7
Underwriting discounts and commissions and offering expenses	$ 900	$ 6,400
Common Stock | Underwriters
Class Of Stock [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares		2,142,857
Period granted to underwriters to purchase additional shares		30 days

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Activities Under Company's Stock Option Plans (Details)	3 Months Ended
	Mar. 31, 2021$ / sharesshares
Number of Options
Balances, beginning of the period | shares	10,929,530
Options granted | shares	2,792,347
Options exercised | shares	(322,507)
Option forfeited/expired | shares	(1,535,118)
Balances, end of the period | shares	11,864,252
Options exercisable, end of the period | shares	5,341,720
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period | $ / shares	$ 10.77
Options granted | $ / shares	7.87
Options exercised | $ / shares	3.17
Option forfeited/expired | $ / shares	11.15
Balances, end of the period | $ / shares	10.25
Options exercisable, end of the period | $ / shares	$ 12.11

Stock-Based Compensation - Tota

Stock-Based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Stock-based compensation expense:
Stock-based compensation expense	$ 3,034	$ 4,013
Research and development
Stock-based compensation expense:
Stock-based compensation expense	1,337	1,919
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 1,697	$ 2,094

Leases - Additional Information

Leases - Additional Information (Details)	Dec. 10, 2015USD ($)ft²	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)		Dec. 31, 2019USD ($)
Operating Leased Assets [Line Items]
Restricted cash		$ 917,000	$ 917,000	$ 917,000	[1]	$ 917,000
Rent expense		1,300,000	$ 1,300,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount		900,000
Restricted cash		$ 900,000		$ 900,000
New Lease Agreement
Operating Leased Assets [Line Items]
Area of rentable office and laboratory space | ft²	76,000
Lease term description		The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease.
Initial lease term	10 years
Extended lease term	5 years
Maximum one-time improvement allowance	$ 12,600,000
Improvement allowance from recoverable rent	2,300,000
New Lease Agreement | First year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	3,100,000
New Lease Agreement | Second twelve months of lease term
Operating Leased Assets [Line Items]
Annual lease rent	4,300,000
New Lease Agreement | Tenth year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	$ 5,500,000
[1]	The condensed balance sheet as of December 31, 2020 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

Leases - Summary of Supplementa

Leases - Summary of Supplemental Information Related to Leases (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$ 1,273	$ 1,239
Supplemental balance sheet information related to leases:
Operating lease right-of-use assets	21,736		$ 22,495	[1]
Current operating lease liabilities	3,297		3,195
Non-current operating lease liabilities	$ 20,807		$ 21,675	[1]
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent		us-gaap:OtherLiabilitiesCurrent
Total operating lease liabilities	$ 24,104		$ 24,870
Weighted-average remaining lease term (in years)
Operating lease	5 years 6 months		5 years 9 months
Weighted-average discount rate
Operating lease	8.25%		8.25%
[1]	The condensed balance sheet as of December 31, 2020 was derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020.

Leases - Schedule of Maturity o

Leases - Schedule of Maturity of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Maturity of operating lease liabilities
2021	$ 3,856
2022	5,273
2023	5,420
2024	5,572
2025 and beyond	10,116
Total lease payments	30,237
Less imputed interest	(6,133)
Present value of lease liabilities	$ 24,104	$ 24,870

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - Patent Infringement Lawsuit - USD ($)	Mar. 04, 2020	Mar. 31, 2021
Loss Contingencies [Line Items]
Lawsuit filed date	March 4, 2020
Name of plaintiff		Vytacera Bio, LLC
Loss contingency, answer, affirmative defenses and counterclaims filing date	May 26, 2020
Loss contingency claim amount		$ 0
Loss contingency, answer and counterclaims by plaintiff filing date	Jun. 5, 2020

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details)	Mar. 27, 2020
Income Tax Disclosure [Abstract]
Net operating loss carryback	5 years
Percentage of taxable income from net operating losses for suspension of annual deduction limitation	80.00%